This invention generally relates to lock nuts designed to have low on/high off characteristics, as well as relates to devices and methods for providing that a lock nut has low on/high off torque characteristics. More specifically, the present invention relates to a crimped lock nut, and a device and method for crimping a lock nut, such that the crimped lock nut has low on/high off torque characteristics.
In many applications, such as in automobile applications, it is desirable to provide that a lock nut has low on/high off torque characteristics (i.e., the lock nut is easy to thread on, but resists threading off or is difficult to remove).
In the past, one way in which lock nuts have been provided with high torque off characteristics is through a Stover crimp. In a Stover crimping process, flats are applied to either two or three sides of a lock nut, and the flats are pushed toward the axial center of the lock nut. The pushing causes the geometry of the threaded bore in the lock nut to distort. While the distortion provides that the lock nut has higher torque off characteristics, the distortion provides that the contact and torque between the lock nut and the structure on which the lock nut is to be threaded (hereinafter assumed to be a threaded bolt) is not optimum—i.e., due to narrow contact points. The distortion may result in the lock nut having such high torque requirements that the threaded bolt is subject to breaking (i.e., due to galling between the lock nut and threaded bolt) as the lock nut is installed or removed. Crimping using flats may also cause burrs to form on the lock nut.
Another way in which lock nuts have been provided with high torque off characteristics is by providing a nylon insert in the lock nut. While lock nuts with nylon inserts typically have slightly better on-torque to off-torque ratios than do lock nuts without nylon inserts, the fact that the inserts are nylon provides that such lock nuts are not practical for use in hot applications—i.e., in applications where the nylon insert will be subject to a significant amount of heat, as the nylon inserts are subject to melting in such an environment.
In order to hold the nylon insert, a lock nut must be formed with a barrel at one end. As such, lock nuts which are designed for use with nylon inserts are typically more difficult to manufacture (i.e., require more assembly, more man hours, and more floor space to manufacture) than are lock nuts which are not designed for use with nylon inserts. Furthermore, nylon inserts are subject to popping out during assembly.
Still another way in which lock nuts have been provided with high torque off characteristics is through a cone crimp. In a cone crimping process, a cone is applied to the top of the lock nut in order to collapse the threads at the top of the nut. While this generally increases the torque required to install or remove the nut, it also causes distortion of the thread pitch. The distortion may result in the lock nut having such high torque requirements that the threaded bolt is subject to breaking (i.e., due to galling between the lock nut and threaded bolt) as the lock nut is installed or removed. Typically, with regard to a cone crimp, the on-torque to off-torque range is not as forgiving. In other words, the on torque must be great to provide a significant off torque. As such, any slight variation may cause galling, low off torque, or broken bolts.